Powders are processed in a wide variety of industries, including cement, chemicals, food, plastics and coal-fired powerplants, to name a few. The facilities where the powders are processed or stored commonly require conveyors to transport them around the plant. The powders are usually conveyed pneumatically, which is preferred to the alternative, mechanical conveying, because of its versatility and cleanliness.
Pneumatic conveyers provide pipes into which the powders are injected and transported by airflow. The powder is normally transferred into the pipe from an unpressurized bin. To prevent air leakage into the bin, a continuously-operating checkvalve is provided that allows the powder to enter the pipe but prevents the backflow of air into the bin; such a valve is called an airlock.
The most commonly used airlock, called a rotary valve, includes a rotor revolving inside a casing. Shaped like a revolving door, but spinning on a horizontal axis, rotary valves allow the powder to enter through an opening in the top of the casing, be moved to the assembly's bottom by the action of the rotor, and then fall out through an opening at the casing's bottom.
Rotary valves are built with a narrow gap between the rotor and casing to keep the rotor from seizing up due to thermal expansion caused by the temperature between the casing and the rotor due to the movement of the rotor relative to the casing. The gap creates a problem when the powder being conveyed is fine and abrasive. The pressure in the conveying line entrains the powder and drives it through the gap at velocities upwards of 700 miles per hour, causing severe erosion if the powders are fine enough to flow through the gaps and are even minimally abrasive. In such applications, the life of rotary valves is too short to be useful.
An alternative airlock design, the screwpump, is used under these conditions. With a configuration similar to a domestic meatgrinder, screwpumps compress the powder by the action of a variable-pitch screw turning inside a casing. The compressed powder forms the seal that prevents the backflow of air. The electrical power needed to compress the powder with a screwpump is relatively high compared with rotary valves. Even with this limitation, screwpumps are limited to certain materials. Very abrasive materials cause excessive wear, and materials without sufficient fines or containing a lot of coarse fractions won't compact sufficiently to form a good seal.
Another airlock, the eductor, is used to convey fine and abrasive powders if the conveying line is at low pressure. Like the screwpump, the eductor normally provides acceptable levels of reliability but also uses much more electricity than a rotary valve.
Because of their low power consumption, rotary valves are still preferred in applications where low pressure and/or abrasiveness provide acceptable levels of reliability. To minimize erosion, the gap between the rotor and casing must be made as small as possible.
Gaps between the rotor and casing of conventional rotary valves occur in three areas: at the blade tips (at the rotor's outside diameter); at the blade ends (at the blades' edges in the axial direction) and in the corners (between the blade tips and the blade ends). Several designs are employed to minimize leakage.
To minimize leakage at the blade tips, the end of each blade in some rotary valves is fitted with a foil that bends back to form a seal as the blade enters the casing. To create a tight seal, the foil's springiness must be made stiff enough to counteract the airlock's back pressure. On the return side of the airlock (where the blades are moving upwards), the back pressure works in the opposite direction, adding to the spring loading and producing high bearing stresses at the tip of the foil. This quickly wears out the foils, making them ineffective after a short time. As a result, flexible foils aren't widely used.
An alternative design uses adjustable seals that are fitted as closely to the casing as possible and then fastened in place. This overcomes the effect of manufacturing tolerances and rotor wear, but still requires some clearance between the rotor and the casing to avoid seizure due to thermal expansion. The use of adjustable seals reduces the clearances.
A further alternative design uses pivoted shoes mounted inside the top of the casing near the top of the rotor. The shoes are made to fit the curvature of the rotor and are spring-loaded to provide a tight seal even when the rotor expands due to thermal expansion. The shoes are made of hardened materials to further reduce wear. This design is limited by thermal expansion which causes the curvature of the shoe to differ from that of the rotor, causing a small gap that becomes the source of erosion.
To minimize air leakage and erosion at the blade ends between the rotor and the end plates of the casing, the rotor is enclosed with end-disks that are made integral with the blades. This method is only partially effective. Leakage still occurs between the end-disks and the casing, causing erosion there.
The air leakage can be further reduced by introducing purge-air into the casing, which displaces the dust-laden air in the space between the casing's end plates and the rotor's end-disks. The purge-air increases the blowby of air into the rotary valve's inlet, which limits the purge-air's usefulness. To overcome this, yet another modification is sometimes made: seals are mounted at the tips of the rotor disks. But these seals wear quickly because the full pressure differential of the airlock presses the seal into the end-disks, at least over part of the perimeter. This causes the seals to wear very quickly.
There is currently no effective way to minimize leakage at the corner between the blade tips and the blade ends. Devices, such as adjustable seals or spring-loaded shoes can't be extended all the way to the corners or seizure will occur. The spaces left in the corners thus become a source of erosion.
Rotary valves made for handling abrasive powders are made from the hardest materials available, generally cast alloys with a Brinnel hardness of over 600. Despite this precaution, and the use of the abrasion-reducing design elements described above, failure can occur in a short time. For example, a rotary valve used to inject 200-mesh limestone at the relatively moderate pressure of 6 psi into a pneumatic conveying line at a coal-fired powerplant is found to fail abruptly and catastrophically after about 12 weeks of operation. Reliable operation requires rebuilding the airlock every eight weeks and replacing it every eight months.
The rotary valves' life is found to be independent of the design: the same results were obtained with the highest-quality abrasion-resistant rotary valves made by three manufacturers. Such a high level of maintenance, in conjunction with the high cost of the airlocks, makes rotary valves marginally useful in this application. Other powerplants, using coarser limestone and/or limestone with less silica, have found rotary valves wearing out slower than what was experienced above.
Erosion is also caused by "rotor crunch", the entrapment of solids that have fallen onto the blades at the rotary valve's inlet and are pinched between the rotor blades and the casing as the blade leaves the inlet opening. Sufficiently hard particles gouge the casing, providing a path for erosion to start and grow. Rotor crunch is also objectionable in some applications, such as the conveying of plastic pellets, because of the damage it does to the pellets.
Another significant problem of rotary valves is blowby. This is the escape of air from various sources into the rotary valve's inlet. Even with non-abrasive powders, blowby can sufficiently interfere with the flow of the incoming solids to render the rotary valves nonoperational. In this event, other types of airlocks such as eductors must be used, despite their much-higher operating costs. The severity of the problem is directly related to the amount of blowby, the fineness of the powders, and their adhesive properties. With sticky solids such as wet coal fines, and even dry but slightly-sticky solids such as carbon black, the turbulence in the inlet piping caused by blowby creates wall buildups that eventually shut off the inlet. Even with freeflowing fine powders, sufficiently-high blowby airflow will completely block the flow of the incoming solids.
Vents are used with conventional rotary valves to direct the pressurized air in the return-side pockets away from the inlet. Vents on the supply side would fill with solids and thus are not feasible because air leaking through the supply side of the airlock would end up in the inlet, causing blowby.
To minimize the effect of blowby, hoppers are frequently installed at the rotary valve's inlet that allow the solids to enter on one side of the hopper while most of the blowby air leaves at the other. This is only marginally successful in most cases. The best cure is to reduce the amount of blowby airflow.
Another problem of conventional rotary valves is their pressure limit. Under most circumstances, rotary valves are limited to differential pressures of under 15 psi. Even with nonabrasive materials and reinforced construction, rotary valves are normally limited to an operating pressure of 40 psi, even though many applications for airlocks occur at higher pressures. Higher pressure causes the rotary valve's shaft to bend, which has to withstand the force created by the airlock pressure across the rotor's entire cross-section. For reasonably-sized shafts, at pressures over 40 psi, shaft bending would require a significantly enlarged gap between the blade and the casing to prevent seizure, which would increase blowby and reduce performance.
Rotary valves are also temperature limited. The higher the temperature of the incoming solids, the greater the potential temperature difference between the rotors and the casing, and thus the larger the potential gap needed to avoid seizure. As a result, lockhoppers are normally used instead of rotary valves in high temperature airlock applications. Lockhoppers are even more subject to abrasive wear than are rotary valves, and therefore are limited to even lower pressures. Rotary valves used as letdown devices, such as in coal-ash removal systems, whereby the pressure at the inlet is higher than the outlet, are similarly limited in their temperature capability.
Finally, conventional rotary valves are subject to pluggage. Fine powders such as carbon black, as well as damp materials such as wet coal, wedge themselves into the corners of the pockets, disabling the airlock by filling the pockets. Some powders are free-flowing enough to eventually fall from the pockets, but the release from the walls is slow enough to reduce the airlock's capacity.
To avoid pluggage, some rotary valves use a so-called blowthrough design instead of the more customary dropthrough designs. In the dropthrough design, the pneumatic conveying pipe is located beneath the rotary valve. In the blowthrough design, the pneumatic air passes through the rotary valve itself, entering and leaving through holes in the bottom of the casing's end plates. The velocity of the pneumatic air stream is used to dislodge the solids.
The blowthrough design can't be employed with closed-rotor rotary valves because the rotor's end disks intercept the pneumatic airflow. But closed-end rotors are required with abrasive solids. The blowthrough design is also of limited use with very sticky solids, such as wet coal or clay because the pneumatic conveying air's velocity isn't sufficient to dislodge such materials.
Rotary valves may be operated as either feeders or airlocks. Feeders are used to meter the flow of solids. When operated as a feeder, the standpipe above the rotary valve's inlet is maintained full of solids, and the pockets of the rotary valve are always full of solids. When operated as an airlock the rotary valve is operated at a higher throughput capacity than the rate of incoming solids, so its pockets are partially empty. The rotary valve's throughput capacity is controlled by its size and rotational speed.
Operation of a rotary valve with its pockets full of solids greatly increases the chances of rotor crunch. Operating the rotary valve as a feeder, and then depending on its sealing characteristics as an airlock, isn't feasible with abrasive materials or in applications where damage to the particles is unacceptable.